[PATCH v8 0/4] mm/swap, memcg: Introduce swap tiers for cgroup based swap control

[PATCH v8 0/4] mm/swap, memcg: Introduce swap tiers for cgroup based swap control

[Youngjun Park]

This is the v8 series of the swap tier patchset.

Great thanks to Shakeel Butt and Yosry for the reviews and discussions [1].
The main change in this version is the interface change to use
memory.swap.tiers.max with '0' (disable) and 'max' (enable) values.
This mechanism was suggested by Shakeel and Yosry.

This change allows for future extensions to control swap
between tiers and aligns better with existing memcg interfaces.
Even with this memcg interface change, only patch #3 needed updates.
Internally, patch #3 still uses the existing mask processing method
(which is implementation-efficient), so only the user-facing interface
was modified.

We also discussed tier extensions. Thanks to Yosry, Nhat and Shakeel for their
valuable feedback.

Here is a brief summary of our tentative conclusions. Please correct me
if anything is misrepresented (details in references):

* Zswap tiering [2]:
  Tiering applies only to the vswap + zswap combo. Zswap itself will
  not be tiered, as the current architecture requires a physical device
  for zswap allocation.
* Vswap tiering [3]:
  Vswap should be handled transparently to the user. Vswap itself will
  not be tiered. But, someday supported if there is strong and real usecase.
* Relationship with zswap.writeback [4]:
  If zswap tiering is introduced, it could replace the zswap-only tier.
  However, since zswap cannot be tiered independently, it is still
  needed for non-vswap cases. Separately, the internal logic could
  potentially be integrated into the tiering logic.
* Tier demotion [5]:
  A separate interface like memory.swap.tiers.demotion might be needed.
  For now, we only support 0/max to enable/disable tiers. In the future,
  we could introduce an "auto" mode to automatically scale the limit
  based on swapfile size and memory.swap.max, similar to the direction
  memory tiering is heading in.

I plan to apply the swap tier infrastructure and the first use case
(cgroup-based swap control) first, and continue following up on the
discussions above.

Overview
========

Swap Tiers group swap devices into performance classes (e.g. NVMe,
HDD, Network) and allow per-memcg selection of which tiers to use.
This mechanism was suggested by Chris Li.

Design Rationale
================

Swap tier selection is attached to memcg. A child cgroup may select a
subset of the parent's allowed tiers.

This
- Preserves cgroup inheritance semantics (boundary at parent,
  refinement at child).
- Reuses memcg, which already groups processes and enforces
  hierarchical memory limits.
- Aligns with existing memcg swap controls (e.g. swap.max, zswap.writeback)
- Avoids introducing a parallel swap control hierarchy.

Placing tier control outside memcg (e.g., via BPF, syscalls, or
madvise) would allow swap preference to diverge from the memcg
hierarchy. Integrating it into memcg keeps the swap policy
consistent with existing memory ownership semantics. There are
also real use cases built around memcg.

In the future, this can be extended to other interfaces to cover
additional use cases.

I believe a memcg-based swap control is a good starting point
before such extensions.

Use Cases
=========

#1: Latency separation (our primary deployment scenario)
  [ / ]
     |
     +-- latency-sensitive workload  (fast tier)
     +-- background workload         (slow tier)

The parent defines the memory boundary.
Each workload selects a swap tier via memory.swap.tiers.max according to
latency requirements.

This prevents latency-sensitive workloads from being swapped to
slow devices used by background workloads.

#2: Per-VM swap selection (Chris Li's deployment scenario)
  [ / ]
     |
     +-- [ Job on VM ]              (tiers: zswap, SSD)
            |
            +-- [ VMM guest memory ]  (tiers: SSD)

The parent (job) has access to both zswap and SSD tiers.
The child (VMM guest memory) selects SSD as its swap tier via
memory.swap.tiers.max. In this deployment, swap device selection
happens at the child level from the parent's available set.

#3: Tier isolation for reduced contention (hypothetical)
  [ / ]                    (tiers: A, B)
     |
     +-- workload X        (tiers: A)
     +-- workload Y        (tiers: B)

Each child uses a different tier. Since swap paths are separated
per tier, synchronization overhead between the two workloads is
reduced.

Future extension
================

#1: Intra-tier distribution policy:
  Currently, swap devices with the same priority are allocated in a
  round-robin fashion. Per-tier policy files under
  /sys/kernel/mm/swap/tiers/ can control how devices within a tier
  are selected (e.g. round-robin, weighted).

#2: Inter-tier promotion and demotion:
  Promotion and demotion apply between tiers, not within a single
  tier. The current interface defines only tier assignment; it does
  not yet define when or how pages move between tiers. Two triggering
  models are possible:

  (a) User-triggered: userspace explicitly initiates migration between
      tiers (e.g. via a new interface or existing move_pages semantics).
  (b) Kernel-triggered: the kernel moves pages between tiers at
      appropriate points such as reclaim or refault.

#3: Per-VMA, per-process swap and BPF:
  Not just for memcg based swap, possible to extend Per-VMA or per-process
  swap. Or we can use it as BPF program.

#4: Zswap and vswap tiering:
  Tiering applies to the vswap + zswap combination.

#5: Vswap on/off control:
  Currently not supported. If a strong use case arises where vswap needs
  to be controlled by memcg, the tier interface could be used for it.

Experimentation
===============

Tested on our internal platform using NBD as a separate swap tier.
Our first production's simple usecase.

Without tiers:
- No selective control over flash wear
- Cannot selectively assign NBD to specific applications

Cold launch improvement (preloaded vs. baseline):
- App A: 13.17s -> 4.18s (68%)
- App B: 5.60s -> 1.12s (80%)
- App C: 10.25s -> 2.00s (80%)

Performance impact with no tiers configured:
<1% regression in kernel build and vm-scalability benchmarks

Change log
===========

v8
- Changed the memcg interface to memory.swap.tiers.max.
  Values are '0' (disable) and 'max' (enable). Default is 'max'.
- Addressed Sashiko's review: Update the mask value atomically at once and
  read the mask value while grabbing lock.
- Collected review tags from Kairui and Nhat.
- Rebase on recent mm-new
- v7 link: https://lore.kernel.org/linux-mm/20260527062247.3440692-1-youngjun.park@lge.com/

v7
- Collect Baoquan's review tag
- Baoquan's feedback on fixing improper comment
- Minor code adjustments per Baoquan's feedback.
- Rebase on recent mm-new
- v6 link: https://lore.kernel.org/linux-mm/20260421055323.940344-1-youngjun.park@lge.com/

v6
- Sashiko AI review fixes
 - Fix batch parsing error path to restore snapshot before exit
 - Reject overlong tier names to prevent truncated duplicates
 - Avoid restoring raw list_head via memcpy (stale pointer risk)
 - Ensure early parse errors do not skip DEF_SWAP_PRIO validation
 - Use (1U << TIER_DEFAULT_IDX) to avoid signed shift UB
 - Defer tier mask inheritance to css_online() to close race window
 - Add READ_ONCE()/WRITE_ONCE() for tier mask accesses
- Other fixes
 - Fix build error reintroduced due to missing v5 change
 - Fix WARNING in folio_tier_effective_mask by adding rcu_read_lock()
 - default number of swap tier max (change to 32->31, for reserving last bit)
 - commit message refinement.
 - rebased on recently mm-new
- v5 link: https://lore.kernel.org/linux-mm/20260325175453.2523280-1-youngjun.park@lge.com/

v5
- Fixed build errors reported in v4
- rebased on up to date mm-new
- Minor cleanups
- Design docs with validation (by Shakeel Butt discussion)
- v4 link : https://lore.kernel.org/linux-mm/20260217000950.4015880-1-youngjun.park@lge.com/

v4
- Simplified control flow and indentation
- Added CONFIG option for MAX_SWAPTIER (default: 4)
- Added memory.swap.tiers.effective interface
- Reworked save/restore logic into snapshot/rollback model
- Removed tier priority modification support (deferred)
- Improved validation and fixed edge cases
- Rebased onto latest mm-new
- RFC v3 link: https://lore.kernel.org/linux-mm/20260131125454.3187546-1-youngjun.park@lge.com/

RFC v1 ~ v3
- Change the direction after discussion with Chris-Li
- apply some LPC feedback.
- RFC v2 - https://lore.kernel.org/linux-mm/20260126065242.1221862-1-youngjun.park@lge.com/
- RFC v1 - https://lore.kernel.org/linux-mm/20251109124947.1101520-1-youngjun.park@lge.com/

Earlier Approach (per cgroup swap priority)
- v1: https://lore.kernel.org/linux-mm/20250716202006.3640584-1-youngjun.park@lge.com/
- RFC: https://lore.kernel.org/linux-mm/aEvLjEInMQC7hEyh@yjaykim-PowerEdge-T330/T/#mbbb6a5e9e30843097e1f5f65fb98f31d582b973d

Reference
=========

[1] https://lore.kernel.org/linux-doc/aiw2p5ANjsQUCIHA@linux.dev/
[2] https://lore.kernel.org/linux-mm/CAKEwX=Nz9SWcEVQGQjHN8P8OANJY4BG0w+iQOzoNOWuteoVjAg@mail.gmail.com/
[3] https://lore.kernel.org/cgroups/CAKEwX=O23a4iWBZoewKVb8QqODte6r3Xijckw3_oCJNoiO9M5A@mail.gmail.com/
[4] https://lore.kernel.org/linux-mm/CAO9r8zOg0OP1Ak1v7CRzSfQq0D8b4Dw+_T0Jui6YTM_KwQQNOA@mail.gmail.com/
[5] https://lore.kernel.org/linux-mm/CAO9r8zNi4-QC4sUi=xXWHt9WMeG39mbyoSf8kON9vLOZ=cbCmw@mail.gmail.com/

Youngjun Park (4):
  mm: swap: introduce swap tier infrastructure
  mm: swap: associate swap devices with tiers
  mm: memcontrol: add interface for swap tier selection
  mm: swap: filter swap allocation by memcg tier mask

 Documentation/admin-guide/cgroup-v2.rst |  20 +
 Documentation/mm/index.rst              |   1 +
 Documentation/mm/swap-tier.rst          | 159 ++++++++
 MAINTAINERS                             |   3 +
 include/linux/memcontrol.h              |   5 +
 include/linux/swap.h                    |   1 +
 mm/Kconfig                              |  12 +
 mm/Makefile                             |   2 +-
 mm/memcontrol.c                         |  67 ++++
 mm/swap.h                               |   4 +
 mm/swap_state.c                         |  75 ++++
 mm/swap_tier.c                          | 483 ++++++++++++++++++++++++
 mm/swap_tier.h                          |  76 ++++
 mm/swapfile.c                           |  20 +-
 14 files changed, 923 insertions(+), 5 deletions(-)
 create mode 100644 Documentation/mm/swap-tier.rst
 create mode 100644 mm/swap_tier.c
 create mode 100644 mm/swap_tier.h

base-commit: 9d335aed8840f6bf83ba93309ae5e185de829c21
-- 
2.34.1

[PATCH v8 1/4] mm: swap: introduce swap tier infrastructure

[Youngjun Park]

This patch introduces the "Swap tier" concept, which serves as an
abstraction layer for managing swap devices based on their performance
characteristics (e.g., NVMe, HDD, Network swap).

Swap tiers are user-named groups representing priority ranges.
Tier names must consist of alphanumeric characters and underscores.
These tiers collectively cover the entire priority space from -1
(`DEF_SWAP_PRIO`) to `SHRT_MAX`.

To configure tiers, a new sysfs interface is exposed at
/sys/kernel/mm/swap/tiers. The input parser evaluates commands from
left to right and supports batch input, allowing users to add or remove
multiple tiers in a single write operation.

Tier management enforces continuous priority ranges anchored by start
priorities. Operations trigger range splitting or merging, but overwriting
start priorities is forbidden. Merging expands lower tiers upwards to
preserve configured start priorities, except when removing `DEF_SWAP_PRIO`,
which merges downwards.

Suggested-by: Chris Li <chrisl@kernel.org>
Reviewed-by: Baoquan He <baoquan.he@linux.dev>
Signed-off-by: Youngjun Park <youngjun.park@lge.com>

diff --git a/MAINTAINERS b/MAINTAINERS
index 65bd4328fe05..d1bb3b4b1e1c 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -17060,6 +17060,8 @@ F:	mm/swap.c
 F:	mm/swap.h
 F:	mm/swap_table.h
 F:	mm/swap_state.c
+F:	mm/swap_tier.c
+F:	mm/swap_tier.h
 F:	mm/swapfile.c
 
 MEMORY MANAGEMENT - THP (TRANSPARENT HUGE PAGE)
diff --git a/mm/Kconfig b/mm/Kconfig
index 776b67c66e82..5343937f3da9 100644
--- a/mm/Kconfig
+++ b/mm/Kconfig
@@ -19,6 +19,18 @@ menuconfig SWAP
 	  used to provide more virtual memory than the actual RAM present
 	  in your computer.  If unsure say Y.
 
+config NR_SWAP_TIERS
+        int "Number of swap device tiers"
+        depends on SWAP
+        default 4
+        range 1 31
+        help
+          Sets the number of swap device tiers. Swap devices are
+          grouped into tiers based on their priority, allowing the
+          system to prefer faster devices over slower ones.
+
+          If unsure, say 4.
+
 config ZSWAP
 	bool "Compressed cache for swap pages"
 	depends on SWAP
diff --git a/mm/Makefile b/mm/Makefile
index eff9f9e7e061..29cb1e778285 100644
--- a/mm/Makefile
+++ b/mm/Makefile
@@ -75,7 +75,7 @@ ifdef CONFIG_MMU
 	obj-$(CONFIG_ADVISE_SYSCALLS)	+= madvise.o
 endif
 
-obj-$(CONFIG_SWAP)	+= page_io.o swap_state.o swapfile.o
+obj-$(CONFIG_SWAP)	+= page_io.o swap_state.o swapfile.o swap_tier.o
 obj-$(CONFIG_ZSWAP)	+= zswap.o
 obj-$(CONFIG_HAS_DMA)	+= dmapool.o
 obj-$(CONFIG_HUGETLBFS)	+= hugetlb.o hugetlb_sysfs.o hugetlb_sysctl.o
diff --git a/mm/swap.h b/mm/swap.h
index 77d2d14eda42..d6c5f5d31f63 100644
--- a/mm/swap.h
+++ b/mm/swap.h
@@ -34,6 +34,10 @@ extern int page_cluster;
 #define swap_entry_order(order)	0
 #endif
 
+#define DEF_SWAP_PRIO  -1
+
+extern spinlock_t swap_lock;
+extern struct plist_head swap_active_head;
 extern struct swap_info_struct *swap_info[];
 
 /*
diff --git a/mm/swap_state.c b/mm/swap_state.c
index 9c3a5cf99778..762d9ca6ad5a 100644
--- a/mm/swap_state.c
+++ b/mm/swap_state.c
@@ -25,6 +25,7 @@
 #include "internal.h"
 #include "swap_table.h"
 #include "swap.h"
+#include "swap_tier.h"
 
 /*
  * swapper_space is a fiction, retained to simplify the path through
@@ -1007,8 +1008,81 @@ static ssize_t vma_ra_enabled_store(struct kobject *kobj,
 }
 static struct kobj_attribute vma_ra_enabled_attr = __ATTR_RW(vma_ra_enabled);
 
+static ssize_t tiers_show(struct kobject *kobj,
+				     struct kobj_attribute *attr, char *buf)
+{
+	return swap_tiers_sysfs_show(buf);
+}
+
+static ssize_t tiers_store(struct kobject *kobj,
+			    struct kobj_attribute *attr,
+			    const char *buf, size_t count)
+{
+	char *p, *token, *name, *tmp;
+	int ret = 0;
+	short prio;
+
+	tmp = kstrdup(buf, GFP_KERNEL);
+	if (!tmp)
+		return -ENOMEM;
+
+	spin_lock(&swap_lock);
+	spin_lock(&swap_tier_lock);
+	swap_tiers_snapshot();
+
+	p = tmp;
+	while ((token = strsep(&p, ", \t\n")) != NULL) {
+		if (!*token)
+			continue;
+
+		switch (token[0]) {
+		case '+':
+			name = token + 1;
+			token = strchr(name, ':');
+			if (!token) {
+				ret = -EINVAL;
+				goto restore;
+			}
+			*token++ = '\0';
+			if (kstrtos16(token, 10, &prio)) {
+				ret = -EINVAL;
+				goto restore;
+			}
+			ret = swap_tiers_add(name, prio);
+			if (ret)
+				goto restore;
+			break;
+		case '-':
+			ret = swap_tiers_remove(token + 1);
+			if (ret)
+				goto restore;
+			break;
+		default:
+			ret = -EINVAL;
+			goto restore;
+		}
+	}
+
+	if (!swap_tiers_validate()) {
+		ret = -EINVAL;
+		goto restore;
+	}
+	goto out;
+
+restore:
+	swap_tiers_snapshot_restore();
+out:
+	spin_unlock(&swap_tier_lock);
+	spin_unlock(&swap_lock);
+	kfree(tmp);
+	return ret ? ret : count;
+}
+
+static struct kobj_attribute tier_attr = __ATTR_RW(tiers);
+
 static struct attribute *swap_attrs[] = {
 	&vma_ra_enabled_attr.attr,
+	&tier_attr.attr,
 	NULL,
 };
 
diff --git a/mm/swap_tier.c b/mm/swap_tier.c
new file mode 100644
index 000000000000..ac7a3c2a48cb
--- /dev/null
+++ b/mm/swap_tier.c
@@ -0,0 +1,302 @@
+// SPDX-License-Identifier: GPL-2.0
+#include <linux/swap.h>
+#include <linux/memcontrol.h>
+#include "memcontrol-v1.h"
+#include <linux/sysfs.h>
+#include <linux/plist.h>
+
+#include "swap.h"
+#include "swap_tier.h"
+
+#define MAX_SWAPTIER	CONFIG_NR_SWAP_TIERS
+#define MAX_TIERNAME	16
+
+/*
+ * struct swap_tier - structure representing a swap tier.
+ *
+ * @name: name of the swap_tier.
+ * @prio: starting value of priority.
+ * @list: linked list of tiers.
+ */
+static struct swap_tier {
+	char name[MAX_TIERNAME];
+	short prio;
+	struct list_head list;
+} swap_tiers[MAX_SWAPTIER];
+
+DEFINE_SPINLOCK(swap_tier_lock);
+/* active swap priority list, sorted in descending order */
+static LIST_HEAD(swap_tier_active_list);
+/* unused swap_tier object */
+static LIST_HEAD(swap_tier_inactive_list);
+
+#define TIER_IDX(tier)	((tier) - swap_tiers)
+#define TIER_MASK(tier)	(1U << TIER_IDX(tier))
+#define TIER_INACTIVE_PRIO (DEF_SWAP_PRIO - 1)
+#define TIER_IS_ACTIVE(tier) ((tier->prio) !=  TIER_INACTIVE_PRIO)
+#define TIER_END_PRIO(tier) \
+	(!list_is_first(&(tier)->list, &swap_tier_active_list) ? \
+	list_prev_entry((tier), list)->prio - 1 : SHRT_MAX)
+
+#define for_each_tier(tier, idx) \
+	for (idx = 0, tier = &swap_tiers[0]; idx < MAX_SWAPTIER; \
+		idx++, tier = &swap_tiers[idx])
+
+#define for_each_active_tier(tier) \
+	list_for_each_entry(tier, &swap_tier_active_list, list)
+
+#define for_each_inactive_tier(tier) \
+	list_for_each_entry(tier, &swap_tier_inactive_list, list)
+
+/*
+ * Naming Convention:
+ *   swap_tiers_*() - Public/exported functions
+ *   swap_tier_*()  - Private/internal functions
+ */
+
+static bool swap_tier_is_active(void)
+{
+	return !list_empty(&swap_tier_active_list);
+}
+
+static struct swap_tier *swap_tier_lookup(const char *name)
+{
+	struct swap_tier *tier;
+
+	for_each_active_tier(tier) {
+		if (!strcmp(tier->name, name))
+			return tier;
+	}
+
+	return NULL;
+}
+
+/* Insert new tier into the active list sorted by priority. */
+static void swap_tier_activate(struct swap_tier *new)
+{
+	struct list_head *pos = &swap_tier_active_list;
+	struct swap_tier *tier;
+
+	for_each_active_tier(tier) {
+		if (tier->prio <= new->prio) {
+			pos = &tier->list;
+			break;
+		}
+	}
+
+	list_add_tail(&new->list, pos);
+}
+
+static void swap_tier_inactivate(struct swap_tier *tier)
+{
+	list_move(&tier->list, &swap_tier_inactive_list);
+	tier->prio = TIER_INACTIVE_PRIO;
+}
+
+void swap_tiers_init(void)
+{
+	struct swap_tier *tier;
+	int idx;
+
+	BUILD_BUG_ON(BITS_PER_TYPE(int) < MAX_SWAPTIER);
+
+	for_each_tier(tier, idx) {
+		INIT_LIST_HEAD(&tier->list);
+		swap_tier_inactivate(tier);
+	}
+}
+
+ssize_t swap_tiers_sysfs_show(char *buf)
+{
+	struct swap_tier *tier;
+	ssize_t len = 0;
+
+	len += sysfs_emit_at(buf, len, "%-16s %-5s %-11s %-11s\n",
+			 "Name", "Idx", "PrioStart", "PrioEnd");
+
+	spin_lock(&swap_tier_lock);
+	for_each_active_tier(tier) {
+		len += sysfs_emit_at(buf, len, "%-16s %-5td %-11d %-11d\n",
+				     tier->name,
+				     TIER_IDX(tier),
+				     tier->prio,
+				     TIER_END_PRIO(tier));
+	}
+	spin_unlock(&swap_tier_lock);
+
+	return len;
+}
+
+static struct swap_tier *swap_tier_prepare(const char *name, short prio)
+{
+	struct swap_tier *tier;
+
+	lockdep_assert_held(&swap_tier_lock);
+
+	if (prio < DEF_SWAP_PRIO)
+		return ERR_PTR(-EINVAL);
+
+	if (list_empty(&swap_tier_inactive_list))
+		return ERR_PTR(-ENOSPC);
+
+	tier = list_first_entry(&swap_tier_inactive_list,
+		struct swap_tier, list);
+
+	list_del_init(&tier->list);
+	strscpy(tier->name, name, MAX_TIERNAME);
+	tier->prio = prio;
+
+	return tier;
+}
+
+static int swap_tier_check_range(short prio)
+{
+	struct swap_tier *tier;
+
+	lockdep_assert_held(&swap_lock);
+	lockdep_assert_held(&swap_tier_lock);
+
+	for_each_active_tier(tier) {
+		/* No overwrite */
+		if (tier->prio == prio)
+			return -EINVAL;
+	}
+
+	return 0;
+}
+
+static bool swap_tier_validate_name(const char *name)
+{
+	int len;
+
+	if (!name || !*name)
+		return false;
+
+	len = strlen(name);
+	if (len >= MAX_TIERNAME)
+		return false;
+
+	while (*name) {
+		if (!isalnum(*name) && *name != '_')
+			return false;
+		name++;
+	}
+	return true;
+}
+
+int swap_tiers_add(const char *name, int prio)
+{
+	int ret;
+	struct swap_tier *tier;
+
+	lockdep_assert_held(&swap_lock);
+	lockdep_assert_held(&swap_tier_lock);
+
+	/* Duplicate check */
+	if (swap_tier_lookup(name))
+		return -EEXIST;
+
+	if (!swap_tier_validate_name(name))
+		return -EINVAL;
+
+	ret = swap_tier_check_range(prio);
+	if (ret)
+		return ret;
+
+	tier = swap_tier_prepare(name, prio);
+	if (IS_ERR(tier)) {
+		ret = PTR_ERR(tier);
+		return ret;
+	}
+
+	swap_tier_activate(tier);
+
+	return ret;
+}
+
+int swap_tiers_remove(const char *name)
+{
+	int ret = 0;
+	struct swap_tier *tier;
+
+	lockdep_assert_held(&swap_lock);
+	lockdep_assert_held(&swap_tier_lock);
+
+	tier = swap_tier_lookup(name);
+	if (!tier)
+		return -EINVAL;
+
+	/* Removing DEF_SWAP_PRIO merges into the higher tier. */
+	if (!list_is_singular(&swap_tier_active_list)
+		&& tier->prio == DEF_SWAP_PRIO)
+		list_prev_entry(tier, list)->prio = DEF_SWAP_PRIO;
+
+	swap_tier_inactivate(tier);
+
+	return ret;
+}
+
+static struct swap_tier swap_tiers_snap[MAX_SWAPTIER];
+/*
+ * XXX: When multiple operations (adds and removes) are submitted in a
+ * single write, reverting each individually on failure is complex and
+ * error-prone. Instead, snapshot the entire state beforehand and
+ * restore it wholesale if any operation fails.
+ */
+void swap_tiers_snapshot(void)
+{
+	BUILD_BUG_ON(sizeof(swap_tiers_snap) != sizeof(swap_tiers));
+
+	lockdep_assert_held(&swap_lock);
+	lockdep_assert_held(&swap_tier_lock);
+
+	memcpy(swap_tiers_snap, swap_tiers, sizeof(swap_tiers));
+}
+
+void swap_tiers_snapshot_restore(void)
+{
+	struct swap_tier *tier;
+	int idx;
+
+	lockdep_assert_held(&swap_lock);
+	lockdep_assert_held(&swap_tier_lock);
+
+	memcpy(swap_tiers, swap_tiers_snap, sizeof(swap_tiers));
+
+	INIT_LIST_HEAD(&swap_tier_active_list);
+	INIT_LIST_HEAD(&swap_tier_inactive_list);
+
+	/*
+	 * memcpy copied snapshot-time list pointers into each tier's
+	 * list_head.  Those references are stale, so re-init every
+	 * tier before re-linking into the freshly initialised global
+	 * lists below.
+	 */
+	for_each_tier(tier, idx) {
+		INIT_LIST_HEAD(&tier->list);
+
+		if (TIER_IS_ACTIVE(tier))
+			swap_tier_activate(tier);
+		else
+			swap_tier_inactivate(tier);
+	}
+}
+
+bool swap_tiers_validate(void)
+{
+	struct swap_tier *tier;
+
+	/*
+	 * Initial setting might not cover DEF_SWAP_PRIO.
+	 * Swap tier must cover the full range (DEF_SWAP_PRIO to SHRT_MAX).
+	 */
+	if (swap_tier_is_active()) {
+		tier = list_last_entry(&swap_tier_active_list,
+			struct swap_tier, list);
+
+		if (tier->prio != DEF_SWAP_PRIO)
+			return false;
+	}
+
+	return true;
+}
diff --git a/mm/swap_tier.h b/mm/swap_tier.h
new file mode 100644
index 000000000000..a1395ec02c24
--- /dev/null
+++ b/mm/swap_tier.h
@@ -0,0 +1,20 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _SWAP_TIER_H
+#define _SWAP_TIER_H
+
+#include <linux/types.h>
+#include <linux/spinlock.h>
+
+extern spinlock_t swap_tier_lock;
+
+/* Initialization and application */
+void swap_tiers_init(void);
+ssize_t swap_tiers_sysfs_show(char *buf);
+
+int swap_tiers_add(const char *name, int prio);
+int swap_tiers_remove(const char *name);
+
+void swap_tiers_snapshot(void);
+void swap_tiers_snapshot_restore(void);
+bool swap_tiers_validate(void);
+#endif /* _SWAP_TIER_H */
diff --git a/mm/swapfile.c b/mm/swapfile.c
index e3d126602a1e..3f7225dbc6cd 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -48,6 +48,7 @@
 #include "swap_table.h"
 #include "internal.h"
 #include "swap.h"
+#include "swap_tier.h"
 
 static void swap_range_alloc(struct swap_info_struct *si,
 			     unsigned int nr_entries);
@@ -63,7 +64,8 @@ static void move_cluster(struct swap_info_struct *si,
  *
  * Also protects swap_active_head total_swap_pages, and the SWP_WRITEOK flag.
  */
-static DEFINE_SPINLOCK(swap_lock);
+DEFINE_SPINLOCK(swap_lock);
+
 static unsigned int nr_swapfiles;
 atomic_long_t nr_swap_pages;
 /*
@@ -74,7 +76,6 @@ atomic_long_t nr_swap_pages;
 EXPORT_SYMBOL_GPL(nr_swap_pages);
 /* protected with swap_lock. reading in vm_swap_full() doesn't need lock */
 long total_swap_pages;
-#define DEF_SWAP_PRIO  -1
 unsigned long swapfile_maximum_size;
 #ifdef CONFIG_MIGRATION
 bool swap_migration_ad_supported;
@@ -87,7 +88,7 @@ static const char Bad_offset[] = "Bad swap offset entry ";
  * all active swap_info_structs
  * protected with swap_lock, and ordered by priority.
  */
-static PLIST_HEAD(swap_active_head);
+PLIST_HEAD(swap_active_head);
 
 /*
  * all available (active, not full) swap_info_structs
@@ -3988,6 +3989,7 @@ static int __init swapfile_init(void)
 		swap_migration_ad_supported = true;
 #endif	/* CONFIG_MIGRATION */
 
+	swap_tiers_init();
 	return 0;
 }
 subsys_initcall(swapfile_init);
-- 
2.34.1

[PATCH v8 2/4] mm: swap: associate swap devices with tiers

[Youngjun Park]

This patch connects swap devices to the swap tier infrastructure,
ensuring that devices are correctly assigned to tiers based on their
priority.

A `tier_mask` is added to identify the tier membership of swap devices.
Although tier-based allocation logic is not yet implemented, this
mapping is necessary to track which tier a device belongs to. Upon
activation, the device is assigned to a tier by matching its priority
against the configured tier ranges.

The infrastructure allows dynamic modification of tiers, such as
splitting or merging ranges. These operations are permitted provided
that the tier assignment of already configured swap devices remains
unchanged.

This patch also adds the documentation for the swap tier feature,
covering the core concepts, sysfs interface usage, and configuration
details.

Reviewed-by: Baoquan He <baoquan.he@linux.dev>
Signed-off-by: Youngjun Park <youngjun.park@lge.com>

diff --git a/Documentation/mm/index.rst b/Documentation/mm/index.rst
index 7aa2a8886908..a0d1447c5569 100644
--- a/Documentation/mm/index.rst
+++ b/Documentation/mm/index.rst
@@ -21,6 +21,7 @@ see the :doc:`admin guide <../admin-guide/mm/index>`.
    page_reclaim
    swap
    swap-table
+   swap-tier
    page_cache
    shmfs
    oom
diff --git a/Documentation/mm/swap-tier.rst b/Documentation/mm/swap-tier.rst
new file mode 100644
index 000000000000..0fb4a1153a67
--- /dev/null
+++ b/Documentation/mm/swap-tier.rst
@@ -0,0 +1,150 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+:Author: Chris Li <chrisl@kernel.org> Youngjun Park <youngjun.park@lge.com>
+
+==========
+Swap Tier
+==========
+
+Swap tier is a collection of user-named groups classified by priority ranges.
+It acts as a facilitation layer, allowing users to manage swap devices based
+on their speeds.
+
+Users are encouraged to assign swap device priorities according to device
+speed to fully utilize this feature. While the current implementation is
+integrated with cgroups, the concept is designed to be extensible for other
+subsystems in the future.
+
+Priority Range
+--------------
+
+The specified tiers must cover the entire priority range from -1
+(DEF_SWAP_PRIO) to SHRT_MAX.
+
+Consistency
+-----------
+
+Tier consistency is guaranteed with a focus on maximizing flexibility. When a
+swap device is activated within a tier range, the tier covering that device's
+priority is guaranteed not to disappear or change while the device remains
+active. Adding a new tier may split the range of an existing tier, but the
+active device's tier assignment remains unchanged.
+
+However, specifying a tier in a cgroup does not guarantee the tier's existence.
+Consequently, the corresponding tier can disappear at any time.
+
+Configuration Interface
+-----------------------
+
+The swap tiers can be configured via the following interface:
+
+/sys/kernel/mm/swap/tiers
+
+Operations can be performed using the following syntax:
+
+* Add:    ``+"<tiername>":"<start_priority>"``
+* Remove: ``-"<tiername>"``
+
+Tier names must consist of alphanumeric characters and underscores. Multiple
+operations can be provided in a single write, separated by commas (",") or
+whitespace (spaces, tabs, newlines).
+
+When configuring tiers, the specified value represents the **start priority**
+of that tier. The end priority is automatically determined by the start
+priority of the next higher tier. Consequently, adding a tier
+automatically adjusts the ranges of adjacent tiers to ensure continuity.
+
+Examples
+--------
+
+**1. Initialization**
+
+A tier starting at -1 is mandatory to cover the entire priority range up to
+SHRT_MAX. In this example, 'HDD' starts at 50, and 'NET' covers the remaining
+lower range starting from -1.
+
+::
+
+    # echo "+HDD:50, +NET:-1" > /sys/kernel/mm/swap/tiers
+    # cat /sys/kernel/mm/swap/tiers
+    Name             Idx   PrioStart   PrioEnd
+    HDD              0     50          32767
+    NET              1     -1          49
+
+**2. Adding a New Tier (split)**
+
+A new tier 'SSD' is added at priority 100, splitting the existing 'HDD' tier.
+The ranges are automatically recalculated:
+
+* 'SSD' takes the top range (100 to SHRT_MAX).
+* 'HDD' is adjusted to the range between 'NET' and 'SSD' (50 to 99).
+* 'NET' remains unchanged (-1 to 49).
+
+::
+
+    # echo "+SSD:100" > /sys/kernel/mm/swap/tiers
+    # cat /sys/kernel/mm/swap/tiers
+    Name             Idx   PrioStart   PrioEnd
+    SSD              2     100         32767
+    HDD              0     50          99
+    NET              1     -1          49
+
+**3. Removal (merge)**
+
+Tiers can be removed using the '-' prefix.
+::
+
+    # echo "-SSD" > /sys/kernel/mm/swap/tiers
+
+When a tier is removed, its priority range is merged into the adjacent
+tier. The merge direction is always upward (the tier below expands),
+except when the lowest tier is removed — in that case the tier above
+shifts its starting priority down to -1 to maintain full range coverage.
+
+::
+
+    Initial state:
+    Name             Idx   PrioStart   PrioEnd
+    SSD              2     100         32767
+    HDD              1     50          99
+    NET              0     -1          49
+
+    # echo "-SSD" > /sys/kernel/mm/swap/tiers
+
+    Name             Idx   PrioStart   PrioEnd
+    HDD              1     50          32767       <- merged with SSD's range
+    NET              0     -1          49
+
+    # echo "-NET" > /sys/kernel/mm/swap/tiers
+
+    Name             Idx   PrioStart   PrioEnd
+    HDD              1     -1          32767       <- shifted down to -1
+
+**4. Interaction with Active Swap Devices**
+
+If a swap device is active (swapon), the tier covering that device's
+priority cannot be removed. Splitting the active tier's range is only
+allowed above the device's priority.
+
+Assume a swap device is active at priority 60 (inside 'HDD' tier).
+
+::
+
+    # swapon -p 60 /dev/zram0
+
+    Name             Idx   PrioStart   PrioEnd
+    HDD              0     50          32767
+    NET              1     -1          49
+
+    # echo "-HDD" > /sys/kernel/mm/swap/tiers
+    -bash: echo: write error: Device or resource busy
+
+    # echo "+SSD:60" > /sys/kernel/mm/swap/tiers
+    -bash: echo: write error: Device or resource busy
+
+    # echo "+SSD:100" > /sys/kernel/mm/swap/tiers
+
+    Name             Idx   PrioStart   PrioEnd
+    SSD              2     100         32767
+    HDD              0     50          99          <- device (prio 60) stays here
+    NET              1     -1          49
diff --git a/MAINTAINERS b/MAINTAINERS
index d1bb3b4b1e1c..4293048be1ab 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -17052,6 +17052,7 @@ L:	linux-mm@kvack.org
 S:	Maintained
 F:	Documentation/ABI/testing/sysfs-kernel-mm-swap
 F:	Documentation/mm/swap-table.rst
+F:	Documentation/mm/swap-tier.rst
 F:	include/linux/swap.h
 F:	include/linux/swapfile.h
 F:	include/linux/swapops.h
diff --git a/include/linux/swap.h b/include/linux/swap.h
index 6d72778e6cc3..21286945770a 100644
--- a/include/linux/swap.h
+++ b/include/linux/swap.h
@@ -250,6 +250,7 @@ struct swap_info_struct {
 	struct percpu_ref users;	/* indicate and keep swap device valid. */
 	unsigned long	flags;		/* SWP_USED etc: see above */
 	signed short	prio;		/* swap priority of this type */
+	int tier_mask;			/* swap tier mask */
 	struct plist_node list;		/* entry in swap_active_head */
 	signed char	type;		/* strange name for an index */
 	unsigned int	max;		/* size of this swap device */
diff --git a/mm/swap_state.c b/mm/swap_state.c
index 762d9ca6ad5a..2f382d4dcbdc 100644
--- a/mm/swap_state.c
+++ b/mm/swap_state.c
@@ -1063,7 +1063,7 @@ static ssize_t tiers_store(struct kobject *kobj,
 		}
 	}
 
-	if (!swap_tiers_validate()) {
+	if (!swap_tiers_update()) {
 		ret = -EINVAL;
 		goto restore;
 	}
diff --git a/mm/swap_tier.c b/mm/swap_tier.c
index ac7a3c2a48cb..6b57cadb3e95 100644
--- a/mm/swap_tier.c
+++ b/mm/swap_tier.c
@@ -38,6 +38,8 @@ static LIST_HEAD(swap_tier_inactive_list);
 	(!list_is_first(&(tier)->list, &swap_tier_active_list) ? \
 	list_prev_entry((tier), list)->prio - 1 : SHRT_MAX)
 
+#define MASK_TO_TIER(mask) (&swap_tiers[__ffs((mask))])
+
 #define for_each_tier(tier, idx) \
 	for (idx = 0, tier = &swap_tiers[0]; idx < MAX_SWAPTIER; \
 		idx++, tier = &swap_tiers[idx])
@@ -59,6 +61,26 @@ static bool swap_tier_is_active(void)
 	return !list_empty(&swap_tier_active_list);
 }
 
+static bool swap_tier_prio_in_range(struct swap_tier *tier, short prio)
+{
+	if (tier->prio <= prio && TIER_END_PRIO(tier) >= prio)
+		return true;
+
+	return false;
+}
+
+static bool swap_tier_prio_is_used(short prio)
+{
+	struct swap_tier *tier;
+
+	for_each_active_tier(tier) {
+		if (tier->prio == prio)
+			return true;
+	}
+
+	return false;
+}
+
 static struct swap_tier *swap_tier_lookup(const char *name)
 {
 	struct swap_tier *tier;
@@ -99,6 +121,7 @@ void swap_tiers_init(void)
 	int idx;
 
 	BUILD_BUG_ON(BITS_PER_TYPE(int) < MAX_SWAPTIER);
+	BUILD_BUG_ON(MAX_SWAPTIER > TIER_DEFAULT_IDX);
 
 	for_each_tier(tier, idx) {
 		INIT_LIST_HEAD(&tier->list);
@@ -149,17 +172,29 @@ static struct swap_tier *swap_tier_prepare(const char *name, short prio)
 	return tier;
 }
 
-static int swap_tier_check_range(short prio)
+static int swap_tier_can_split_range(short new_prio)
 {
+	struct swap_info_struct *p;
 	struct swap_tier *tier;
 
 	lockdep_assert_held(&swap_lock);
 	lockdep_assert_held(&swap_tier_lock);
 
-	for_each_active_tier(tier) {
-		/* No overwrite */
-		if (tier->prio == prio)
-			return -EINVAL;
+	plist_for_each_entry(p, &swap_active_head, list) {
+		if (p->tier_mask == TIER_DEFAULT_MASK)
+			continue;
+
+		tier = MASK_TO_TIER(p->tier_mask);
+		if (!swap_tier_prio_in_range(tier, new_prio))
+			continue;
+
+		/*
+		 * Device sits in a tier that spans new_prio;
+		 * splitting here would reassign it to a
+		 * different tier.
+		 */
+		if (p->prio >= new_prio)
+			return -EBUSY;
 	}
 
 	return 0;
@@ -199,7 +234,11 @@ int swap_tiers_add(const char *name, int prio)
 	if (!swap_tier_validate_name(name))
 		return -EINVAL;
 
-	ret = swap_tier_check_range(prio);
+	/* No overwrite */
+	if (swap_tier_prio_is_used(prio))
+		return -EBUSY;
+
+	ret = swap_tier_can_split_range(prio);
 	if (ret)
 		return ret;
 
@@ -226,6 +265,11 @@ int swap_tiers_remove(const char *name)
 	if (!tier)
 		return -EINVAL;
 
+	/* Simulate adding a tier to check for conflicts */
+	ret = swap_tier_can_split_range(tier->prio);
+	if (ret)
+		return ret;
+
 	/* Removing DEF_SWAP_PRIO merges into the higher tier. */
 	if (!list_is_singular(&swap_tier_active_list)
 		&& tier->prio == DEF_SWAP_PRIO)
@@ -236,13 +280,15 @@ int swap_tiers_remove(const char *name)
 	return ret;
 }
 
-static struct swap_tier swap_tiers_snap[MAX_SWAPTIER];
 /*
- * XXX: When multiple operations (adds and removes) are submitted in a
- * single write, reverting each individually on failure is complex and
- * error-prone. Instead, snapshot the entire state beforehand and
- * restore it wholesale if any operation fails.
+ * XXX: Static global snapshot buffer for batch operations. Small
+ * and used once per write, so a static global is not bad.
+ * When multiple adds/removes are submitted in a single write,
+ * reverting each individually on failure is error-prone. Instead,
+ * snapshot beforehand and restore wholesale if any operation fails.
  */
+static struct swap_tier swap_tiers_snap[MAX_SWAPTIER];
+
 void swap_tiers_snapshot(void)
 {
 	BUILD_BUG_ON(sizeof(swap_tiers_snap) != sizeof(swap_tiers));
@@ -282,10 +328,30 @@ void swap_tiers_snapshot_restore(void)
 	}
 }
 
-bool swap_tiers_validate(void)
+void swap_tiers_assign_dev(struct swap_info_struct *swp)
 {
 	struct swap_tier *tier;
 
+	lockdep_assert_held(&swap_lock);
+
+	for_each_active_tier(tier) {
+		if (swap_tier_prio_in_range(tier, swp->prio)) {
+			swp->tier_mask = TIER_MASK(tier);
+			return;
+		}
+	}
+
+	swp->tier_mask = TIER_DEFAULT_MASK;
+}
+
+bool swap_tiers_update(void)
+{
+	struct swap_tier *tier;
+	struct swap_info_struct *swp;
+
+	lockdep_assert_held(&swap_lock);
+	lockdep_assert_held(&swap_tier_lock);
+
 	/*
 	 * Initial setting might not cover DEF_SWAP_PRIO.
 	 * Swap tier must cover the full range (DEF_SWAP_PRIO to SHRT_MAX).
@@ -298,5 +364,16 @@ bool swap_tiers_validate(void)
 			return false;
 	}
 
+	/*
+	 * If applied initially, the swap tier_mask may change
+	 * from the default value.
+	 */
+	plist_for_each_entry(swp, &swap_active_head, list) {
+		/* Tier is already configured */
+		if (swp->tier_mask != TIER_DEFAULT_MASK)
+			break;
+		swap_tiers_assign_dev(swp);
+	}
+
 	return true;
 }
diff --git a/mm/swap_tier.h b/mm/swap_tier.h
index a1395ec02c24..3e355f857363 100644
--- a/mm/swap_tier.h
+++ b/mm/swap_tier.h
@@ -5,8 +5,15 @@
 #include <linux/types.h>
 #include <linux/spinlock.h>
 
+/* Forward declarations */
+struct swap_info_struct;
+
 extern spinlock_t swap_tier_lock;
 
+#define TIER_ALL_MASK		(~0)
+#define TIER_DEFAULT_IDX	(31)
+#define TIER_DEFAULT_MASK	(1U << TIER_DEFAULT_IDX)
+
 /* Initialization and application */
 void swap_tiers_init(void);
 ssize_t swap_tiers_sysfs_show(char *buf);
@@ -16,5 +23,9 @@ int swap_tiers_remove(const char *name);
 
 void swap_tiers_snapshot(void);
 void swap_tiers_snapshot_restore(void);
-bool swap_tiers_validate(void);
+bool swap_tiers_update(void);
+
+/* Tier assignment */
+void swap_tiers_assign_dev(struct swap_info_struct *swp);
+
 #endif /* _SWAP_TIER_H */
diff --git a/mm/swapfile.c b/mm/swapfile.c
index 3f7225dbc6cd..9a86ebe992f4 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -3036,6 +3036,8 @@ static void _enable_swap_info(struct swap_info_struct *si)
 
 	/* Add back to available list */
 	add_to_avail_list(si, true);
+
+	swap_tiers_assign_dev(si);
 }
 
 /*
-- 
2.34.1

[PATCH v8 3/4] mm: memcontrol: add interface for swap tier selection

[Youngjun Park]

Introduce memory.swap.tiers.max, a flat-keyed file listing each
tier defined in /sys/kernel/mm/swap/tiers with its state, "max"
(allowed, the default) or "0" (disabled).  A tier is one bit in the
cgroup's tier mask, so writing "<tier> max" or "<tier> 0" sets or
clears that bit.

Since the current use case lacks amount control, it only supports
"max" (on) and "0" (off). Therefore, it does not track per-tier swap
usage, relying instead on a fast runtime bitmask check.

We maintain both `mask` and `effective_mask`. The `effective_mask` is
strictly bounded by the parent (e.g., if a parent is "0", the child's
effective state is "0" even if its `mask` is "max"). Maintaining this
separately avoids costly cgroup tree traversals to check ancestors at
runtime.

Suggested-by: Shakeel Butt <shakeel.butt@linux.dev>
Suggested-by: Yosry Ahmed <yosry@kernel.org>
Signed-off-by: Youngjun Park <youngjun.park@lge.com>

diff --git a/Documentation/admin-guide/cgroup-v2.rst b/Documentation/admin-guide/cgroup-v2.rst
index 6efd0095ed99..4843ffcfd110 100644
--- a/Documentation/admin-guide/cgroup-v2.rst
+++ b/Documentation/admin-guide/cgroup-v2.rst
@@ -1850,6 +1850,26 @@ The following nested keys are defined.
 	Swap usage hard limit.  If a cgroup's swap usage reaches this
 	limit, anonymous memory of the cgroup will not be swapped out.
 
+  memory.swap.tiers.max
+	A read-write flat-keyed file which exists on non-root
+	cgroups.  The default is "max" for every tier.
+
+	Limits the swap tiers this cgroup may swap to.  Tiers are
+	defined globally in /sys/kernel/mm/swap/tiers and listed here,
+	one per line. When read, the values are displayed in descending
+	order of the tiers (highest tier first)::
+
+	  <tier_1> max
+	  <tier_2> 0
+	  ...
+
+	Currently, only "max" and "0" are supported. "max" allows the
+	tier, "0" disables it.  Each write sets a single "<tier> max"
+	or "<tier> 0" pair.
+
+	A child may only narrow what its parent allows. A tier an
+	ancestor disabled stays disabled regardless of the value here.
+
   memory.swap.events
 	A read-only flat-keyed file which exists on non-root cgroups.
 	The following entries are defined.  Unless specified
diff --git a/Documentation/mm/swap-tier.rst b/Documentation/mm/swap-tier.rst
index 0fb4a1153a67..addbc495de8c 100644
--- a/Documentation/mm/swap-tier.rst
+++ b/Documentation/mm/swap-tier.rst
@@ -15,6 +15,15 @@ speed to fully utilize this feature. While the current implementation is
 integrated with cgroups, the concept is designed to be extensible for other
 subsystems in the future.
 
+Use case
+---------
+
+Users can perform selective swapping by choosing a swap tier assigned according
+to speed within a cgroup.
+
+For more information on cgroup v2, please refer to
+``Documentation/admin-guide/cgroup-v2.rst``.
+
 Priority Range
 --------------
 
diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
index e1f46a0016fc..d53826c68562 100644
--- a/include/linux/memcontrol.h
+++ b/include/linux/memcontrol.h
@@ -283,6 +283,11 @@ struct mem_cgroup {
 	struct lru_gen_mm_list mm_list;
 #endif
 
+#ifdef CONFIG_SWAP
+	int tier_mask;
+	int tier_effective_mask;
+#endif
+
 #ifdef CONFIG_MEMCG_V1
 	/* Legacy consumer-oriented counters */
 	struct page_counter kmem;		/* v1 only */
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 56cd4af08232..63259576792a 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -68,6 +68,7 @@
 #include <net/ip.h>
 #include "slab.h"
 #include "memcontrol-v1.h"
+#include "swap_tier.h"
 
 #include <linux/uaccess.h>
 
@@ -4244,6 +4245,8 @@ static int mem_cgroup_css_online(struct cgroup_subsys_state *css)
 	refcount_set(&memcg->id.ref, 1);
 	css_get(css);
 
+	swap_tiers_memcg_inherit_mask(memcg);
+
 	/*
 	 * Ensure mem_cgroup_from_private_id() works once we're fully online.
 	 *
@@ -5785,6 +5788,64 @@ static int swap_events_show(struct seq_file *m, void *v)
 	return 0;
 }
 
+static int swap_tier_max_show(struct seq_file *m, void *v)
+{
+	struct mem_cgroup *memcg = mem_cgroup_from_seq(m);
+
+	swap_tiers_mask_show(m, memcg);
+	return 0;
+}
+
+static ssize_t swap_tier_max_write(struct kernfs_open_file *of,
+				char *buf, size_t nbytes, loff_t off)
+{
+	struct mem_cgroup *memcg = mem_cgroup_from_css(of_css(of));
+	char *pos, *name, *val;
+	bool enable;
+	int mask;
+	int ret = 0;
+
+	pos = strstrip(buf);
+	name = strsep(&pos, " \t\n");
+	if (!name || !*name)
+		return -EINVAL;
+	if (pos)
+		pos = skip_spaces(pos);
+	val = strsep(&pos, " \t\n");
+	if (!val || !*val)
+		return -EINVAL;
+	if (pos && *skip_spaces(pos))
+		return -EINVAL;
+
+	if (!strcmp(val, "max"))
+		enable = true;
+	else if (!strcmp(val, "0"))
+		enable = false;
+	else
+		return -EINVAL;
+
+	spin_lock(&swap_tier_lock);
+	mask = swap_tiers_mask_lookup(name);
+	if (!mask) {
+		ret = -EINVAL;
+		goto out;
+	}
+
+	/*
+	 * tier_mask is set per memcg here; the effective mask is clamped
+	 * to the parent's in swap_tiers_memcg_sync_mask().
+	 */
+	if (enable)
+		WRITE_ONCE(memcg->tier_mask, memcg->tier_mask | mask);
+	else
+		WRITE_ONCE(memcg->tier_mask, memcg->tier_mask & ~mask);
+
+	swap_tiers_memcg_sync_mask(memcg);
+out:
+	spin_unlock(&swap_tier_lock);
+	return ret ? ret : nbytes;
+}
+
 static struct cftype swap_files[] = {
 	{
 		.name = "swap.current",
@@ -5817,6 +5878,12 @@ static struct cftype swap_files[] = {
 		.file_offset = offsetof(struct mem_cgroup, swap_events_file),
 		.seq_show = swap_events_show,
 	},
+	{
+		.name = "swap.tiers.max",
+		.flags = CFTYPE_NOT_ON_ROOT,
+		.seq_show = swap_tier_max_show,
+		.write = swap_tier_max_write,
+	},
 	{ }	/* terminate */
 };
 
diff --git a/mm/swap_state.c b/mm/swap_state.c
index 2f382d4dcbdc..712b225509cc 100644
--- a/mm/swap_state.c
+++ b/mm/swap_state.c
@@ -1021,6 +1021,7 @@ static ssize_t tiers_store(struct kobject *kobj,
 	char *p, *token, *name, *tmp;
 	int ret = 0;
 	short prio;
+	int mask = 0;
 
 	tmp = kstrdup(buf, GFP_KERNEL);
 	if (!tmp)
@@ -1053,7 +1054,7 @@ static ssize_t tiers_store(struct kobject *kobj,
 				goto restore;
 			break;
 		case '-':
-			ret = swap_tiers_remove(token + 1);
+			ret = swap_tiers_remove(token + 1, &mask);
 			if (ret)
 				goto restore;
 			break;
@@ -1063,7 +1064,7 @@ static ssize_t tiers_store(struct kobject *kobj,
 		}
 	}
 
-	if (!swap_tiers_update()) {
+	if (!swap_tiers_update(mask)) {
 		ret = -EINVAL;
 		goto restore;
 	}
diff --git a/mm/swap_tier.c b/mm/swap_tier.c
index 6b57cadb3e95..d6f0e1588dd8 100644
--- a/mm/swap_tier.c
+++ b/mm/swap_tier.c
@@ -253,7 +253,7 @@ int swap_tiers_add(const char *name, int prio)
 	return ret;
 }
 
-int swap_tiers_remove(const char *name)
+int swap_tiers_remove(const char *name, int *mask)
 {
 	int ret = 0;
 	struct swap_tier *tier;
@@ -276,6 +276,7 @@ int swap_tiers_remove(const char *name)
 		list_prev_entry(tier, list)->prio = DEF_SWAP_PRIO;
 
 	swap_tier_inactivate(tier);
+	*mask |= TIER_MASK(tier);
 
 	return ret;
 }
@@ -344,7 +345,26 @@ void swap_tiers_assign_dev(struct swap_info_struct *swp)
 	swp->tier_mask = TIER_DEFAULT_MASK;
 }
 
-bool swap_tiers_update(void)
+#ifdef CONFIG_MEMCG
+static void swap_tier_memcg_propagate(int mask)
+{
+	struct mem_cgroup *child;
+
+	rcu_read_lock();
+	for_each_mem_cgroup_tree(child, root_mem_cgroup) {
+		WRITE_ONCE(child->tier_mask, child->tier_mask | mask);
+		WRITE_ONCE(child->tier_effective_mask,
+			   child->tier_effective_mask | mask);
+	}
+	rcu_read_unlock();
+}
+#else
+static void swap_tier_memcg_propagate(int mask)
+{
+}
+#endif
+
+bool swap_tiers_update(int mask)
 {
 	struct swap_tier *tier;
 	struct swap_info_struct *swp;
@@ -375,5 +395,89 @@ bool swap_tiers_update(void)
 		swap_tiers_assign_dev(swp);
 	}
 
+	/*
+	 * When a tier is removed, its index (bit position in the mask) becomes
+	 * free for reassignment to a future tier. If a memcg had previously
+	 * disabled this tier (cleared the bit in its swap.tiers.max file), the
+	 * effective mask would keep that bit clear -- meaning the new tier at
+	 * the same index would be silently unavailable, an invisible cgroup
+	 * constraint left behind by a tier that no longer exists.
+	 *
+	 * To prevent this, OR the removed tier's mask bit into every memcg's
+	 * tier_mask and tier_effective_mask. This resets the bit so the new
+	 * tier is accessible by default; users who want to restrict it must
+	 * explicitly disable it after the tier is re-created.
+	 */
+	if (mask)
+		swap_tier_memcg_propagate(mask);
+
 	return true;
 }
+
+#ifdef CONFIG_MEMCG
+void swap_tiers_mask_show(struct seq_file *m, struct mem_cgroup *memcg)
+{
+	struct swap_tier *tier;
+	int mask;
+
+	spin_lock(&swap_tier_lock);
+	mask = READ_ONCE(memcg->tier_mask);
+
+	for_each_active_tier(tier)
+		seq_printf(m, "%s %s\n", tier->name,
+			   (mask & TIER_MASK(tier)) ? "max" : "0");
+	spin_unlock(&swap_tier_lock);
+}
+
+int swap_tiers_mask_lookup(const char *name)
+{
+	struct swap_tier *tier;
+
+	lockdep_assert_held(&swap_tier_lock);
+
+	for_each_active_tier(tier) {
+		if (!strcmp(name, tier->name))
+			return TIER_MASK(tier);
+	}
+
+	return 0;
+}
+
+static void __swap_tier_memcg_inherit_mask(struct mem_cgroup *memcg,
+	struct mem_cgroup *parent)
+{
+	int parent_mask = parent
+		? READ_ONCE(parent->tier_effective_mask)
+		: TIER_ALL_MASK;
+
+	WRITE_ONCE(memcg->tier_effective_mask,
+		   parent_mask & READ_ONCE(memcg->tier_mask));
+}
+
+/* Computes the initial effective mask from the parent's effective mask. */
+void swap_tiers_memcg_inherit_mask(struct mem_cgroup *memcg)
+{
+	spin_lock(&swap_tier_lock);
+	rcu_read_lock();
+	memcg->tier_mask = TIER_ALL_MASK;
+	__swap_tier_memcg_inherit_mask(memcg, parent_mem_cgroup(memcg));
+	rcu_read_unlock();
+	spin_unlock(&swap_tier_lock);
+}
+
+/*
+ * Called when a memcg's tier_mask is modified. Walks the subtree
+ * and recomputes each descendant's effective mask against its parent.
+ */
+void swap_tiers_memcg_sync_mask(struct mem_cgroup *memcg)
+{
+	struct mem_cgroup *child;
+
+	lockdep_assert_held(&swap_tier_lock);
+
+	rcu_read_lock();
+	for_each_mem_cgroup_tree(child, memcg)
+		__swap_tier_memcg_inherit_mask(child, parent_mem_cgroup(child));
+	rcu_read_unlock();
+}
+#endif
diff --git a/mm/swap_tier.h b/mm/swap_tier.h
index 3e355f857363..e2f0cf32035b 100644
--- a/mm/swap_tier.h
+++ b/mm/swap_tier.h
@@ -10,22 +10,67 @@ struct swap_info_struct;
 
 extern spinlock_t swap_tier_lock;
 
-#define TIER_ALL_MASK		(~0)
-#define TIER_DEFAULT_IDX	(31)
-#define TIER_DEFAULT_MASK	(1U << TIER_DEFAULT_IDX)
-
 /* Initialization and application */
 void swap_tiers_init(void);
 ssize_t swap_tiers_sysfs_show(char *buf);
 
 int swap_tiers_add(const char *name, int prio);
-int swap_tiers_remove(const char *name);
+int swap_tiers_remove(const char *name, int *mask);
 
 void swap_tiers_snapshot(void);
 void swap_tiers_snapshot_restore(void);
-bool swap_tiers_update(void);
+bool swap_tiers_update(int mask);
 
 /* Tier assignment */
 void swap_tiers_assign_dev(struct swap_info_struct *swp);
 
+#define TIER_ALL_MASK		(~0)
+#define TIER_DEFAULT_IDX	(31)
+#define TIER_DEFAULT_MASK	(1U << TIER_DEFAULT_IDX)
+
+#if defined(CONFIG_SWAP) && defined(CONFIG_MEMCG)
+/* Memcg related functions */
+void swap_tiers_mask_show(struct seq_file *m, struct mem_cgroup *memcg);
+void swap_tiers_memcg_inherit_mask(struct mem_cgroup *memcg);
+void swap_tiers_memcg_sync_mask(struct mem_cgroup *memcg);
+int swap_tiers_mask_lookup(const char *name);
+static inline int folio_tier_effective_mask(struct folio *folio)
+{
+	struct mem_cgroup *memcg;
+	int mask = TIER_ALL_MASK;
+
+	rcu_read_lock();
+	memcg = folio_memcg(folio);
+	if (memcg)
+		mask = READ_ONCE(memcg->tier_effective_mask);
+	rcu_read_unlock();
+
+	return mask;
+}
+#else
+static inline void swap_tiers_mask_show(struct seq_file *m,
+	struct mem_cgroup *memcg) {}
+static inline void swap_tiers_memcg_inherit_mask(struct mem_cgroup *memcg) {}
+static inline void swap_tiers_memcg_sync_mask(struct mem_cgroup *memcg) {}
+static inline int swap_tiers_mask_lookup(const char *name)
+{
+	return 0;
+}
+static inline int folio_tier_effective_mask(struct folio *folio)
+{
+	return TIER_ALL_MASK;
+}
+#endif
+
+/**
+ * swap_tiers_mask_test - Check if the tier mask is valid
+ * @tier_mask: The tier mask to check
+ * @mask: The mask to compare against
+ *
+ * Return: true if condition matches, false otherwise
+ */
+static inline bool swap_tiers_mask_test(int tier_mask, int mask)
+{
+	return tier_mask & mask;
+}
 #endif /* _SWAP_TIER_H */
-- 
2.34.1

[PATCH v8 4/4] mm: swap: filter swap allocation by memcg tier mask

[Youngjun Park]

Apply memcg tier effective mask during swap slot allocation to
enforce per-cgroup swap tier restrictions.

In the fast path, check the percpu cached swap_info's tier_mask
against the folio's effective mask. If it does not match, fall
through to the slow path. In the slow path, skip swap devices
whose tier_mask is not covered by the folio's effective mask.

This works correctly when there is only one non-rotational
device in the system and no devices share the same priority.
However, there are known limitations:

 - When non-rotational devices are distributed across multiple
   tiers, and different memcgs are configured to use those
   distinct tiers, they may constantly overwrite the shared
   percpu swap cache. This cache thrashing leads to frequent
   fast path misses.

 - Combined with the above issue, if same-priority devices exist
   among them, a percpu cache miss (overwritten by another memcg)
   forces the allocator to round-robin to the next device
   prematurely, even if the current cluster is not fully
   exhausted.

These edge cases do not affect the primary use case of
directing swap traffic per cgroup. Further optimization is
planned for future work.

Reviewed-by: Nhat Pham <nphamcs@gmail.com>
Reviewed-by: Kairui Song <kasong@tencent.com>
Reviewed-by: Baoquan He <baoquan.he@linux.dev>
Signed-off-by: Youngjun Park <youngjun.park@lge.com>

diff --git a/mm/swapfile.c b/mm/swapfile.c
index 9a86ebe992f4..1a2d29735b71 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -1365,14 +1365,18 @@ static bool swap_alloc_fast(struct folio *folio)
 	struct swap_cluster_info *ci;
 	struct swap_info_struct *si;
 	unsigned int offset;
+	int mask = folio_tier_effective_mask(folio);
 
 	/*
 	 * Once allocated, swap_info_struct will never be completely freed,
 	 * so checking it's liveness by get_swap_device_info is enough.
 	 */
 	si = this_cpu_read(percpu_swap_cluster.si[order]);
+	if (!si || !swap_tiers_mask_test(si->tier_mask, mask))
+		return false;
+
 	offset = this_cpu_read(percpu_swap_cluster.offset[order]);
-	if (!si || !offset || !get_swap_device_info(si))
+	if (!offset || !get_swap_device_info(si))
 		return false;
 
 	ci = swap_cluster_lock(si, offset);
@@ -1392,10 +1396,14 @@ static bool swap_alloc_fast(struct folio *folio)
 static void swap_alloc_slow(struct folio *folio)
 {
 	struct swap_info_struct *si, *next;
+	int mask = folio_tier_effective_mask(folio);
 
 	spin_lock(&swap_avail_lock);
 start_over:
 	plist_for_each_entry_safe(si, next, &swap_avail_head, avail_list) {
+		if (!swap_tiers_mask_test(si->tier_mask, mask))
+			continue;
+
 		/* Rotate the device and switch to a new cluster */
 		plist_requeue(&si->avail_list, &swap_avail_head);
 		spin_unlock(&swap_avail_lock);
-- 
2.34.1

Re: [PATCH v8 3/4] mm: memcontrol: add interface for swap tier selection

[YoungJun Park]

Sahiko review

https://sashiko.dev/#/message/20260617053447.2831896-4-youngjun.park%40lge.com

I think this might be an overconcern. In normal
situations, cgroup delegation is strictly under the administrator's control.

Furthermore, if a user already has a delegated root, there are numerous
other ways they could potentially cause a DoS attack. Therefore, I believe
this doesn't need to be modified.

Re: [PATCH v8 4/4] mm: swap: filter swap allocation by memcg tier mask

[YoungJun Park]

https://sashiko.dev/#/patchset/20260617053447.2831896-1-youngjun.park@lge.com?part=4

Regarding the review from Sachiko.

The AI review pointed out two things. a pre-existing issue and a valid
issue regarding swap_sync_discard().

For the pre-existing issue, I believe it is out of scope to address it
together in this patchset.

However, the second point is a valid issue. For the case where
swap_sync_discard() discards a non-selected tier, succeeds, and then
retries swap allocation. I will fix this in the next version by
checking the memcg mask within swap_sync_discard().

Re: [PATCH v8 0/4] mm/swap, memcg: Introduce swap tiers for cgroup based swap control

[Nhat Pham]

On Wed, Jun 17, 2026 at 1:34 AM Youngjun Park <youngjun.park@lge.com> wrote:
>
> This is the v8 series of the swap tier patchset.
>
> Great thanks to Shakeel Butt and Yosry for the reviews and discussions [1].
> The main change in this version is the interface change to use
> memory.swap.tiers.max with '0' (disable) and 'max' (enable) values.
> This mechanism was suggested by Shakeel and Yosry

I like this interface too :)

>
> This change allows for future extensions to control swap
> between tiers and aligns better with existing memcg interfaces.
> Even with this memcg interface change, only patch #3 needed updates.
> Internally, patch #3 still uses the existing mask processing method
> (which is implementation-efficient), so only the user-facing interface
> was modified.
>
> We also discussed tier extensions. Thanks to Yosry, Nhat and Shakeel for their
> valuable feedback.
>
> Here is a brief summary of our tentative conclusions. Please correct me
> if anything is misrepresented (details in references):
>
> * Zswap tiering [2]:
>   Tiering applies only to the vswap + zswap combo. Zswap itself will
>   not be tiered, as the current architecture requires a physical device
>   for zswap allocation.

I think Yosry wants zswap as a tier, right?

Just that without vswap, maybe don't allow it to be an tier of itself?

> * Vswap tiering [3]:
>   Vswap should be handled transparently to the user. Vswap itself will
>   not be tiered. But, someday supported if there is strong and real usecase.
> * Relationship with zswap.writeback [4]:
>   If zswap tiering is introduced, it could replace the zswap-only tier.
>   However, since zswap cannot be tiered independently, it is still
>   needed for non-vswap cases. Separately, the internal logic could
>   potentially be integrated into the tiering logic.
> * Tier demotion [5]:
>   A separate interface like memory.swap.tiers.demotion might be needed.
>   For now, we only support 0/max to enable/disable tiers. In the future,
>   we could introduce an "auto" mode to automatically scale the limit
>   based on swapfile size and memory.swap.max, similar to the direction
>   memory tiering is heading in.
>
> I plan to apply the swap tier infrastructure and the first use case
> (cgroup-based swap control) first, and continue following up on the
> discussions above.
>
> Overview
> ========
>
> Swap Tiers group swap devices into performance classes (e.g. NVMe,
> HDD, Network) and allow per-memcg selection of which tiers to use.
> This mechanism was suggested by Chris Li.
>
>
> #2: Inter-tier promotion and demotion:
>   Promotion and demotion apply between tiers, not within a single
>   tier. The current interface defines only tier assignment; it does
>   not yet define when or how pages move between tiers. Two triggering
>   models are possible:
>
>   (a) User-triggered: userspace explicitly initiates migration between
>       tiers (e.g. via a new interface or existing move_pages semantics).
>   (b) Kernel-triggered: the kernel moves pages between tiers at
>       appropriate points such as reclaim or refault.

We'll likely need some kernel-triggered mechanism, or we'd have LRU inversion :)

Cold pages will fill up fast tiers first, and more recent/warm pages
will land on slow tiers...

We'll also need to enforce isolation/fairness to make sure no wordload
hoard the fast tiers too (but that probably requires demotion
support).

>
> #3: Per-VMA, per-process swap and BPF:
>   Not just for memcg based swap, possible to extend Per-VMA or per-process
>   swap. Or we can use it as BPF program.
>
> #4: Zswap and vswap tiering:
>   Tiering applies to the vswap + zswap combination.
>
> #5: Vswap on/off control:
>   Currently not supported. If a strong use case arises where vswap needs
>   to be controlled by memcg, the tier interface could be used for it.

+1.

Also, per-si/per-tier per-CPU allocation caching? :) Kairui already
has a patch for it, IIUC, but if not it's pretty critical I'd say.

BTW, can we add some selftests, to make sure the new interface works
as expected, and to have example programs for new users to model their
scripts after? :)

Re: [PATCH v8 0/4] mm/swap, memcg: Introduce swap tiers for cgroup based swap control

[YoungJun Park]

On Wed, Jun 17, 2026 at 01:50:49PM -0400, Nhat Pham wrote:

> On Wed, Jun 17, 2026 at 1:34 AM Youngjun Park <youngjun.park@lge.com> wrote:
> >
> > This is the v8 series of the swap tier patchset.
> >
> > Great thanks to Shakeel Butt and Yosry for the reviews and discussions [1].
> > The main change in this version is the interface change to use
> > memory.swap.tiers.max with '0' (disable) and 'max' (enable) values.
> > This mechanism was suggested by Shakeel and Yosry
> 
> I like this interface too :)

Good to hear. Now it looks like we have found a memcg interface that
aligns well with the existing memcg model.

I like this idea as well. Thanks again to Shakeel Butt and Yosry.

> > Here is a brief summary of our tentative conclusions. Please correct me
> > if anything is misrepresented (details in references):
> >
> > * Zswap tiering [2]:
> >   Tiering applies only to the vswap + zswap combo. Zswap itself will
> >   not be tiered, as the current architecture requires a physical device
> >   for zswap allocation.
> 
> I think Yosry wants zswap as a tier, right?
> 
> Just that without vswap, maybe don't allow it to be an tier of itself?

With the current architecture, users cannot dynamically specify zswap as
a tier, and zswap is a separate layer, so it is not tiered by itself.

Once your vswap work lands, I think we can make the zswap 
become the default, top-level tier.

After that, we can also look into cleaning up the zswap.writeback
interface together.

> #2: Inter-tier promotion and demotion:
>   Promotion and demotion apply between tiers, not within a single
>   tier. The current interface defines only tier assignment; it does
>   not yet define when or how pages move between tiers. Two triggering
>   models are possible:
>
> >   (a) User-triggered: userspace explicitly initiates migration between
> >       tiers (e.g. via a new interface or existing move_pages semantics).
> >   (b) Kernel-triggered: the kernel moves pages between tiers at
> >       appropriate points such as reclaim or refault.
> 
> We'll likely need some kernel-triggered mechanism, or we'd have LRU inversion :)
> 
> Cold pages will fill up fast tiers first, and more recent/warm pages
> will land on slow tiers...

Yeah, good point!

> We'll also need to enforce isolation/fairness to make sure no wordload
> hoard the fast tiers too (but that probably requires demotion
> support).

Right, that makes sense.

BTW, One thing I am curious about, though, is whether there are strong
real-world use cases that require demotion/promotion.
Theoretically, this looks useful but it would be helpful to better understand 
the requirements from such deployments.

> >
> > #3: Per-VMA, per-process swap and BPF:
> >   Not just for memcg based swap, possible to extend Per-VMA or per-process
> >   swap. Or we can use it as BPF program.
> >
> > #4: Zswap and vswap tiering:
> >   Tiering applies to the vswap + zswap combination.
> >
> > #5: Vswap on/off control:
> >   Currently not supported. If a strong use case arises where vswap needs
> >   to be controlled by memcg, the tier interface could be used for it.
> 
> +1.
> 
> Also, per-si/per-tier per-CPU allocation caching? :) Kairui already
> has a patch for it, IIUC, but if not it's pretty critical I'd say.

Yes, I missed it. Thank you for addressing it.
we need an implementation that integrates this with the per-CPU
allocation currently implemented on the vswap side.

If Kairui's patch lands, my patch #4 also can be optimized based on that.

> BTW, can we add some selftests, to make sure the new interface works
> as expected, and to have example programs for new users to model their
> scripts after? :)

Yes, I agree. I think selftests are necessary.

Do you want them to be introduced in this patchset, or would it be okay
to add them separately as follow-up work?